Is 2nd Gen equipment compatible with new equipment?

SS2 and SS3 components have no cross compatibility. SS2 sensors will only work with a SS2 system and SS3 sensors will only with with a SS3 system.

Except the simplicams, they will work with both SS2 and SS3.  Not sure about the video doorbell, though, maybe someone can chime in  on that.

Second Generation is a risky term.  It would seem that it would refer to SS2, but some people claim it refers to SS3.  So don't rely on that term.  Make sure you know whether it is SS2 or SS3, because SS2 is not encrypted and SS3 is,  So SS3 thinks any SS2 signal is trying to break in, and SS2 thinks any SS3 signal is gobbledegook.